Long-Term Moth Monitoring Program
The larvae of stem-boring moth Dioryctria albovittella
kill the growing stems of pinyons and result in a compact, shrubby
architecture. We currently have over 20 years of census data on
a group of ~175-year-old trees: 20 trees susceptible to moth attack
and 20 trees resistant to moth attack. These trees are intermixed
at the same cinder soil site. We are continuing our annual non-destructive
sampling of shoot mortality caused by the stem-boring moth, and
annually monitor growth and reproduction.
Moth Herbivory:
Differences between trees classified as resistant and susceptible
to moth attack was maintained over about two decades. Susceptible
trees are fed upon 2 to 8 times more frequently than resistant
trees. However, the prolonged drought does seem to be supressing
herbivory on susceptible trees in recent years (Fig.1).
Female Reproduction:
Moth susceptible trees produce fewer cones (Fig.2)
and conelets (Fig.3)
than moth resistant trees. This effect is especially dramatic
during higher cone production years.
Male Reproduction:
Strobili production is generally higher in moth susceptible trees
(Fig.4).
This is not true in another set of 70-year-old trees, where there
is no difference (see moth removal experiment below).
Long-Term Moth Removal Experiment
The larvae of stem-boring moth Dioryctria albovittella
kill the growing stems of pinyons and result in a compact, shrubby
architecture. Since 1982, a group of moth susceptible trees has
been sprayed with insecticide and moths have been effectively
removed. We have monitored these trees, along with control groups
of resistant and susceptible trees for reproduction, herbivory,
and general tree performance. Some of the results from this study
can be found below:
Effectiveness of Herbivore Removal:
Spraying has reduced the amount of herbivory by D. albovittella
on formerly susceptible trees to the same levels as on resistant
trees (Fig.5).
Moth herbivory also has declined in recent years of drought induced
pinyon mortality.
Cone Production:
Moth removal has caused an increase in cone production to levels
comparable (in most years) to those of moth resistant trees (Fig.6).
This effect is most obvious in mast years.
Seed Viability:
Moth resistant trees have a higher percentage of viable seeds
than moth susceptible trees, but removal of moths does not significantly
increase this percentage (Fig.7).
Moths may not directly affect seed viability.
Conelet Production:
Moth removal has increased conelet production (Fig.8).
Strobili Production:
Moth herbivory has no effect on male strobili production for this
set of trees (Fig.9).
In a set of 175 year old trees, moth resistant trees produce more
strobili clusters than moth susceptible trees (see moth monitoring
program above).
Aboveground Productivity:
Moth-removed trees had higher annual stem growth than susceptible
trees in 11 of the 15 years before 2001, while moth resistant
trees had higher growth in only 4 of 15 (Fig.10).
This suggests not only that moths decrease growth, but that there
is also a cost to moth resistance. Since 2001, moth susceptible
trees have generally had the most stem growth, suggesting they
have an advantage during severe drought and/or when moth populations
are supressed. Moth herbivory has no effect on needle litterfall,
which we used as a proxy for litter production (Fig.11).
Cascading Effects On Other Species:
Removal of one herbivore has caused an increase in herbivory by
at least three other organisms – evidence of competitive release
(Fig.12).
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